How long until the ball hits the ground?

[hyde2042]

Homework Statement

A ball is thrown downward from a height of 30m at 8m/s. How long until it hits the ground?
(Air resistance is negligible and gravity is 9.8m/s

Homework Equations

I tried deltaX=Vi+(1/2)a(t^2) and came out with 2.12, but the solution says it's 1.79.

The Attempt at a Solution

30=8+(1/2)(9.8)(t^2)
22=4.9(t^2)
sqrt4.49=sqrt(t^2)
t=2.12

 

[CompuChip]

Are you not missing a t?
$$V = V_i + a t$$
so
$$\Delta x = V_i \mathbb{t} + \frac12 a t^2$$

 

[hyde2042]

Thank you, but I'm afraid I'm evenb more confused by the additional t.

I ended up at 30-8t=4.9(t^2)

What I keep doing after that seems to make it more complicated than it should be. COuld you possibly point me in the right direction? Thank you for your time.

 

[CompuChip]

It's a standard quadratic equation, of the form ax² + bx + c = 0.
How do you solve one of those?

 

[hyde2042]

Ah... hate when I over look simple things. Thank you very much.